Maintenance of embryonic chick duodenum in organ culture in serum-free medium, responsive to vitamin D and with excellent preservation of mucosal structure, has been achieved. It is proposed to further investigate the regulation of synthesis and physiologic role(s) of the vitamin D-induced calcium binding protein (CaBP) in the calcium absorptive mechanism. The precisely controlled in vitro conditions, completely isolated from systemic influences, made possible only through the use of this unique system allows differentiation between direct and indirect effects of various agents or treatments on the intestine itself. Experiments to date have established that vitamin D3 itself and numerous metabolites and analogs, varying over a 10,000-fold range in potency, induce CaBP in this system. Induction of CaBP is paralleled by enhanced transmucosal transport of calcium. Reconstitution experiments have established a role for CaBP in calcium transport. CaBP biosynthesis appears to be regulated in a complex manner by cAMP and calcium ion. Work in progress includes development of a culture medium permitting normal cell proliferation by addition of known (cortisol and thyroid hormone) or suspected proliferogenic agents (other hormones). When "normal" proliferation is achieved, vitamin D-mediated responses will be reevaluated; also, more extensive efforts to study chemical carcinogenesis will be undertaken. Work will continue on the synergistic action of hydrocortisone on 1 alpha, 25-(OH)2-D3-mediated responses, on the role of cAMP in CaBP biosynthesis, on the action of a variety of agents known or suspected to alter calcium absorption in vivo, and on the use of the culture technique in analog assay and as a bioassay for 1 alpha, 25-(OH)2-D3. The obvious utility of the system in assessing the effects of toxic agents (e.g., Cd2 ion and other metals) will be exploited further. The new filter paper method for measuring mucosal cell uptake willl be utilized to study unidirectional fluxes of calcium and to further assess the role of CaBP in calcium transport. Finally, a major effort will be mounted to isolate and purify CaBP-mRNA for reconstitution experiments.